The Effect of Sintering-Redispersion on the Selective Aromatic Yield on Supported Platinum Catalysts

0263–8762/06/$30.00+0.00 # 2006 Institution of Chemical Engineers Trans IChemE, Part A, August 2006 Chemical Engineering Research and Design, 84(A8): 664– 676

www.icheme.org/cherd doi: 10.1205/cherd.04220

THE EFFECT OF SINTERING-REDISPERSION ON THE SELECTIVE AROMATIC YIELD ON SUPPORTED PLATINUM CATALYSTS A. A. SUSU1
, E. O. OGOGO1 and H. M. NGOMO2 2

1 Chemical Engineering Department, University of Lagos, Lagos, Nigeria Department of Inorganic Chemistry, University of Yaounde I, Yaounde, Cameroon

T

he dispersion of supported platinum-containing catalysts was determined by the pulse chemisorption technique. Two catalyst loadings for the monometallic catalyst and a bimetallic catalyst were investigated: 0.3%Pt/Al2O3, 0.6%Pt/Al2O3 and 0.3%Pt – 0.3%Re/Al2O3. Results confirm the usually observed trend that dispersions after oxidation are higher than those after reduction. The redispersion profiles peaked at a sintering temperature of 5508C for all catalysts studied. However, the bimetallic catalyst was found to redisperse at all treatment temperatures while the monometallic catalysts sinter and redisperse at various treatment temperatures. The atomic migration model was proposed as a suitable model for the prediction of the particle size distribution (PSD) as it lends itself easily to the prediction of sintering and redispersion phenomena. The computer simulation for the catalyst resulted in the calculation of dispersion generated PSD, for the oxygen sintered catalyst and the reduced catalyst after sintering. The general PSD profiles for both sintering situations show the same pattern of shift to larger particle sizes after specific temperatures. At a temperature transition of 500 – 5508C, the PSD shifted to lower particle sizes for the 0.3%Pt/Al2O3 and the 0.6%Pt/Al2O3 catalysts. Above 5508C, the PSD shifted to higher particle sizes with larger spreads. On the bimetallic catalyst, the shift of the PSD to the left occurred at two temperature transitions: one at 500 –5508C and the other at 750– 8008C. The selective aromatic yield was studied on a catalyst sintered at various temperatures and reduced at 5008C. At 5008C, toluene was the sole product of the reforming reaction of n-heptane on 0.3%Pt/ Al2O3 catalyst; on the bimetallic catalyst surface, toluene as the sole product of reaction occurred a sintering temperature of 8008C. Although the PSD for the sintered PSD on 0.6%Pt/Al2O3 catalyst was quite similar to that of the 0.3%Pt/Al2O3 catalyst, no aromatization products were formed on this catalyst. Aromatization selectivity was attributed to threeatom ensembles of triangular symmetry of small Pt aggregates (Biloen et al., 1980; Paal, 1980, 2003). If this reaction occurs, it can indicate the presence of facets with (111) symmetry produced during sintering and redispersion. Keywords: Pt-containing catalysts; sintering-redispersion; chemisorption; atomic migration model; aromatic selectivity.

INTRODUCTION AND LITERATURE REVIEW

surmounted by an activation barrier. However, when the activation barrier is overcome by a temperature rise, the catalyst sinters due to loss of surface area and porosity. Sintering is one of the three forms of the deactivation of metal catalysts. Others include poisoning and coking or fouling. In sintering, deactivation results from enhancement of the larger metallic crystallites due to agglomeration of smaller particles leading to the reduction of the metal surface available for reaction. The cause of sintering has been attributed to high temperature during processing and local temperature excursions on the crystallite faces (Steinbruchel and Schmidt, 1973; Ruckenstein and Petty, 1972; Cusumano

Supported metal catalysts used for the reforming of hydrocarbons are usually prepared on high surface area support materials for efficient dispersion of the expensive noble metal (Pt) on the support. This however results in the presence of a thermodynamic driving force that tends to minimize surface free energy, which can only be
Correspondence to: Professor A. A. Susu, Chemical Engineering Department, University of Lagos, Lagos, Nigeria. E-mail: [email protected]

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SINTERING-REDISPERSION ON YIELD OF PLATINUM CATALYSTS and Low, 1970). With specificity to dual functioning reforming catalysts, deactivation is due to losses in the metallic area as a result of crystallite agglomeration and acidity due to chloride loss. Regeneration of supported metal catalysts has been found to be possible depending on the temperature, atmosphere and acid loss. Regeneration schemes centre on the rejuvenation of either the metallic or the acidic sites. The process whereby the sintered catalyst regains its original activity level or surpasses it after regeneration is called redispersion. A number of processes have been formulated industrially for regenerating activated catalyst. Such processes include oxychlorination, carbon burn-off and replacement of support acidity by direct chloride or indirect addition of carbon tetrachloride. Sintering in different atmospheres has different influence on supported metal catalyst. Sintering of Pt/Al2O3 catalyst in hydrogen at temperatures of 500 – 6758C has been found by Dautzenberg and Walters (1978) to produce a decrease in hydrogen uptake in subsequent chemisorption studies. They found that no agglomeration of platinum crystallites took place and observed no redispersion during treatment in oxygen atmosphere. It was stated that restoration of platinum rendered ‘inaccessible’ by previous treatment in hydrogen or sintering was observed. Such inaccessible platinum was reported by Den Otter and Dautzenberg (1978) as resulting from the Pt alloying or complexing with the aluminum of the alumina support. However, it was also reported by the same authors (Dautzenberg and Walters, 1978) that no redispersion occurred at any temperature on treating air-sintered platinum at temperatures above 5008C with oxygen. Yao et al. (1979) have disagreed with the interpretation that the loss of platinum area during heat treatment in hydrogen at 5008C was due to the reduction of the support (alumina) to form PtAl2O3-x complex or a Pt3Al alloy which does not adsorb hydrogen because their temperature programmed reduction studies showed that reduction of platinum oxide is complete at 3008C and that no reduction occurs at 5008C in hydrogen. The use of transmission electron microscopy by Stulgar et al. (1980) for their study on the redispersion behaviour of small platinum particles supported on flat gammaalumina substrates show no significant decrease in platinum particle size when the catalyst was subjected to heat treatment for 18 h at 6008C in air followed by ‘redispersion’ when heated at 5008C for 18 h, also in air. However, their result was not supported by observations of Ruckenstein and Malhortra (1976) who, under similar conditions, found a significant decrease in platinum particle size from 10.7 nm to 4.1 nm by transmission electron microscopy. Redispersion was attributed to two mechanisms: (1) ‘crystallite splitting’ mechanism whereby large crystallites spilt into smaller ones due to a fracture induced by the accumulated strain energy and (2) spreading of the platinum oxide formed on the surface of the support. No conclusive agreement has been reached by the various investigators regarding the mechanism of redispersion during alternate hydrogen and oxygen treatment. There have also been arguments as to the mechanism by which sintering occurs. While Ruckenstein and Chu (1979), Gruber (1962) and Straguzzi et al. (1980) concluded that sintering occurs by crystallite migration, Baker et al.

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(1975) and Chen and Ruckenstein (1981) contend that it is by atomic or molecular migration. However, Chen and Ruckenstein (1981) in their work on the effect of hydrogen treatments on model Pd/Al2O3 catalyst using the electron microscopy concluded that sintering occurs via both the crystallite and atomic migration mechanisms. Ruckenstein and Chu (1979) also reported on a very slow sintering in oxygen compared to sintering in hydrogen on Pt/Al2O3 catalyst. Chen and Schmidt (1978) and Baker et al. (1975) both reported a rapid sintering in oxygen especially at temperatures above 5778C where bulk platinum oxides are unstable. Johnson and Keith (1963) found that heating a commercial platinum alumina catalyst in dry air between 510 – 5808C results in redispersion of the platinum metal while higher temperatures (.5808C) caused sintering of the metal crystallites. However, heating in air at 4808C has also been found to result in redispersion (Jarvoska-Galas and Wrzysz, 1966). Conflicting reports have also arisen with regard to the conditions that promote the timing of sintering and redispersion. During alternate heat treatment in H2 and O2 of supported metal catalysts, Weller and Montagna (1971) observed that the cyclic exposure to oxygen and hydrogen at 5508C results in redispersion of the metal only during reduction and that sintering was observed during oxidation. Ruckenstein and Chu (1979) and Ngomo and Susu (1996) concluded that redispersion occurs during oxidation rather than during reduction. According to Ruckenstein and Chu (1978), sintering occurs during reduction and redispersion during oxidation after several cycles of exposure to hydrogen and oxygen at 7508C. Ngomo and Susu (1996) however, added that the redispersion after oxidation might effectively have commenced during oxidation. The above review exposes the confusion in the literature with regard to, especially, the mechanism of redispersion. Any further research to shed more light on the understanding of this phenomenon would be a worthwhile addition to this very important industrial process. Besides, additional clarification may elucidate further the choice mechanism of the sintering processes. In this publication, we concentrate on two results of our continuing investigation of the monometallic and bimetallic Pt reforming catalysts. The first is on the sintering-redispersion phenomenon on these catalysts and the second is on the effect of this phenomenon on the aromatization selectivity during n-heptane reforming. The reforming catalysts used are: 0.3%Pt/Al2O3, 0.6%Pt/Al2O3 and 0.3%Pt0.3%Pt-Re/Al2O3 catalysts. We therefore looked at the two-fold effect of platinum catalyst loading and the effect of the addition of rhenium to the platinum metal.

THEORETICAL DEVELOPMENT A theoretical development based on the atomic migration model was used to simulate the sintering and redispersion phenomena. The atomic migration model was used because it could predict both the sintering and redispersion data. The atomic migration model has as its main elements the net rate of change of atoms per particle and the rate of change of the number of atoms migrating on the support surface.

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The rate of change of the number of atoms in the ith particle is given by:
 dNi Fs ¼ an Di  AeEa =RT dt Nt So

Ni ¼ 0:5 þ 0:5j1 þ 2qj3 (1)

where a ¼ sticking probability of an atom colliding with a crystallite; v ¼ velocity of migrating atoms ¼ n ¼ (pkBT/2m)1/2; kB ¼ Boltzmann constant; T ¼ absolute temperature; m ¼ mass of migrating atom; So ¼ support area per metal atom; Fs ¼ number of atoms migrating on the support with an area NtSo; Di ¼ effective diameter of crystallite i; Ea ¼ activation energy required to move from a particle to the surface; A ¼ pre-exponential factor. Also, the rate of change of the number of migrating surface atoms is given by M X dFs dNi ¼ dt dt i¼1

¼ MAeEa =RT þ an

M Fs X Di Nt So i¼1

(2)

where M ¼ number of metal crystallites on the support of area NtSo. Solution of equation (1) was presented by Flynn and Wanke (1974a, b, 1975). Equations (1) and (2) could be written in the following finite difference forms for computer solution: 

  F s Ea =RT Di  Ae Dt DNi ¼ an N t So




(3)

and

DF ¼ 

M X

DNi

(4)

i¼1

where DF ¼ arithmetic average of Fs during time increment Dt. For values of (a/So) . 1012 m22 (Apestequia and Barbier, 1982), it was P found that Fs is small, Fs  0 and the approximation M i¼1 DNi  0 could be made. This condition corresponds to rapid rates of surface atom migration and capture to the rate at which atoms leave crystallites. For this case, the rate at which a crystallite captures atoms, assuming all crystallites are equally accessible to migrating atoms, is proportional to diameter, Di (Van Hardeveld and Hartog, 1969). Equations (3) and (4) for this case are reduced to " DNi ¼

! # MDi  1 AeEa =RT Dt PM i¼1 Di

crystallite i, we have the following:

(5)

Assuming the crystallites were fcc cubes and using the equations of Van Hardeveld and Hartog (1969) to determine the total number of atoms, Nsi, at the surface of

2

Ns,i ¼ 12q þ 2

(6) (7)

for q  1 where q ¼ (li 2 ao)/21/2 ao ¼ number of atoms along the edge of the cube minus one; li ¼ length of the edge of cube; ao ¼ atomic number of the metal (0.277 nm for platinum). The effective diameter Di, of a crystallite for the capture of migrating atoms was taken as Di ¼ 4(li þ ao )

(8)

By starting with a given particle size distribution i.e., li for all particles at time t ¼ 0, equation (5) was used to obtain Ni’s as a function of time. The corresponding D was calculated by the relationship: PM Ns,i D ¼ Pi¼1 M i¼1 Ni

(9)

For this model, the particle size distributions (PSD) were first generated. The PSDs were then used to simulate the dispersion at different sintering temperatures by a trial and error process. This simulation was carried out for the 0.3% and the 0.6% Pt/Al2O3 catalyst. We note here the need to show that the use of equation (6) for estimating the number of surface atoms in the platinum crystals is a reasonable one. This is in view of the work of Gnutzmann and Vogel (1990), which has shown, with the use of X-ray diffraction, that the structure of small particles of Pt/SiO2 catalyst was cuboctahedra. In addition, the Dubye function was used in establishing that cubes expose only (100) planes, while cuboctahedra and truncated cuboctahedra expose planes in the (100) and (111) planes by a ration of 2 : 1 and 1 : 3, respectively. Hardeveld and Hartog (1969) gave equations (Poltorak and Boronin, 1966; Van Hardeveld and Van Montfoort, 1966) for the calculations of the number of surface atoms that is dependent on m, the number of atoms lying on an equivalent edge (corner atoms included), for fcc, bcc and hcp crystals. It was also stated that, at low values of m, most surface atoms occur at the corners of the crystal. But at high values of m, nearly all surface atoms are located on the faces of the crystal. For the fcc crystals, the ratios of the number of surface atoms for the cube to cuboctactahedra were about half for high and low values of m. This indicates that although the distribution of the exposed planes is different in the two cases [(100) for the cubic and (100) and (111) for the cuboctahedra], the number of surface atoms present in the fcc cubes will be only about half of that on the fcc cuboctrahedra. Secondly, we had carried out a simulation of the PSD using the cuboctahedra model and found that differences in the PSDs for the two cases were negligible. However, the implications of the cuboctahedra structure and other surface structures on the selectivity of aromatics in the reforming of n-heptane will be discussed appropriately in the text.

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SINTERING-REDISPERSION ON YIELD OF PLATINUM CATALYSTS Particle Size Distribution (PSD) Generation The initial PSD were generated in the following manner: (1) The maximum and minimum size particles (lmax and lmin) in the distribution were chosen. (2) The size range was split into K equal size increments of size Dl. (3) The number of particles, Pk, was specified for each increment. (4) The size of each particle was calculated by use of the relationship:   ( j  1) Dl (10) li ¼ lmin þ (k  1)Dl þ Pk for k ¼ 1 to k þ 1 with po ¼ 0, pk21 ¼ 1 was included to obtain a particle size of lmax. This procedure results in P kþ1 Pk particles all have a different size. Table 1 shows the values of Pk used in the computer program. Estimates of Model Parameters For specific PSD and fixed A, T and (a/So), the dispersion as a function of time is determined by the value of Ae 2Ea/RT, i.e., A ¼ 1.65  107 s21 and Ea/RT ¼ 15.0 (Apestequia and Barbier, 1982). A value of 0.1 h for Dt was used for the reported results. The temperature was increased to the desired temperature over a period of 1 h at 0.1 h increment. EXPERIMENTAL Gases used in this investigation are hydrogen, oxygen and nitrogen, which were all purchased from Industrial Gases, Lagos, Nigeria. Traces of oxygen in the nitrogen gas were removed by passing the gas through reduced cuprous oxide at 3308C. The bed containing the cuprous oxide was reactivated with hydrogen after every 8 h of operation. This time was determined adequate from preliminary experimentation. Three commercial reforming catalysts were also used: 0.3%Pt/Al2O3, 0.6%Pt/Al2O3 and 0.3% –0.3%Pt-Re/Al2O3 catalysts. They were all similar in almost all respects except with regard to their metal and sulfur content. The identical properties are the following: surface area 180 m2 g21 and chlorine content 1%. The pore volume was 0.5 ml g21 on the monometallic catalysts and 0.65 on the bimetallic catalyst while the sulphur

Table 1. Data for the generation of PSD. K

Pk

1 2 3 4 5 6 7 8 9 10

20 20 18 16 14 12 10 7 3 1

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content was 300 ppm on the monometallic catalysts and 50 ppm on the bimetallic catalyst. The experimental set up for the chemisorption studies and the reforming of n-heptane on Pt/Al2O3 and Pt-Re/ Al2O3 catalysts is shown in Figure 1. The microcatalytic or pulse-flow reactor was made of stainless steel (4 mm i.d.) of length 5 cm packed with 150 mg of the catalyst in the size range 53– 75 mm. Both ends of the reactor were plugged with glass wool. A thermocouple was attached to the centre of the reactor wall for monitoring of the reactor’s temperature. The reactor was heated with a vertical Redcroft Stanton furnace. The temperature of the reactor was determined within an accuracy of +18C. A Carle GC was used for the analysis of the exit gases from the reactor. Analysis was done on-line with a split valve. A column packed with 15% FFAP in Chromosorb W.AW containing Carbowax and substituted terephthalic acid was used for the product separation. An alumina column was also used to detect the product gases. The catalysts were dried in a flow of nitrogen at 40 ml min21 and 1108C for 1 h and reduced in a flow of hydrogen at 40 ml min21 and at 5008C for 2 h. A sampling valve was used to ensure a reproducible volume of titrant through the gas chromatograph. The hydrogen uptake was determined by the use of Carle TCD gas chromatograph. Two responses from the TCD were evaluated for each catalyst: (1) the response when the pulse went through the reactor and (2) the response when the same volume of pulse by passes the reactor. Nitrogen was used as carrier and hydrogen was used as tracer. The nitrogen gas was passed through reduced copper oxide to remove traces of oxygen contained therein. Also, both gases were passed through silica gel for the removal of water vapour. The turnover number (TON), defined as the number of molecules of product per sec per number of atoms of exposed metal, is given by

TON ¼

xF C o nW

(11)

where F ¼ flow rate, C¯o ¼ average concentration of reactant, x ¼ conversion, n ¼ number of molecules of exposed platinum atoms and W ¼ weight of platinum catalyst used. All the variables need for the computation of TON is routinely determined with the exception of C¯o which requires knowledge of the pulse shape. A triangular pulse shape was generated and used for the calculation of TON. All subsequent reforming reactions were carried out at 5008C and at a hydrogen carrier gas pressure of 391.9 kPa. In order to maintain the chlorine concentration level, each experimental run did not exceed 24 h as it has been established previously that the chlorine level is unchanged after experimentation for up to 24 h (Straguzzi et al., 1980; Baker et al., 1975; Chen et al., 1979; Chen and Ruckenstein, 1981). We need to ensure that there was no sintering during the experimentation scheme where extended pulsing of more than 70 pulses was injected. Since the time between successive pulses is at least 10 min, the minimum analysis time, the total time the catalyst was exposed at 5008C in hydrogen gas was 700 min or less than 12 h. We have to be sure that no substantial sintering occurred during this period.

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Figure 1. Schematic diagram of the experimental set-up for chemisorption studies and n-heptane reforming on platinum catalysts.

The work of Wanke (1981) on the time dependent sintering of supported platinum catalyst confirmed that no significant dispersion occurred in hydrogen at 6008C for 20 h. Our temperature is lower (5008C) and at a shorter time period (.12 h). The following titration experiments were performed on every single batch of catalyst:

reactor temperature increased again to the desired sintering temperature (between 500 and 8008C). The procedure for the pulsing for chemisorption described above was followed for the determination of the quantity of oxygen adsorbed.

(1) The platinum catalysts were titrated at 258C while the platinum – rhenium catalyst was titrated at both 25 and 5008C. It has been reported that rhenium does not chemisorb hydrogen at 258C. The chemisorption at 258C is due to platinum alone while that at 5008C is due to platinum and rhenium (Wanke, 1981). So, the chemisorption studies at these two temperatures yield the chemisorption of platinum and rhenium separately knowing the stoichiometry of the chemisorption reaction for each metal (Freel, 1972; Wanke, 1981). The platinum catalysts were titrated at 258C while the platinum-rhenium catalysts was titrated at both 258C and 5008C. (2) The catalyst was then treated in oxygen at the particular sintering temperature followed by titration. (3) The sintered catalyst was then reduced in hydrogen for 2 h followed by titration.

RESULTS AND DISCUSSION

A fresh catalyst sample was used for each sintering temperature in the temperature range 500 –8008C. The procedure for oxygen sintering experiments was the same for all the catalysts. Each catalyst batch was dried and reduced as described above. Then, for safety purposes, the reactor temperature was reduced to about 1008C prior to the introduction of oxygen into the system while the flow of hydrogen gas was correspondingly reduced. Then, the oxygen flow rate was then set at 60 ml min21 and the

Chemisorption Pt/Al2O3 catalyst Tables 2 and 3 show the summary of the results for the chemisorption experiments on both 0.3%Pt/Al2O3 and 0.6%Pt/Al2O3 catalysts. Dispersion is defined as the ratio of surface atoms to the total metal atoms in the crystallite. The dispersion on the fresh 0.3%Pt/Al2O3 catalyst was higher (0.24) than on the fresh 0.6%Pt/Al2O3 catalyst (0.19). Also, dispersion was found to be generally higher on the 0.3%Pt/Al2O3 catalyst; the only exception was at 7008C, where dispersion was higher on the 0.6%Pt/Al2O3 catalyst. Figures 2 and 3 show the normalized dispersion profiles as a function of sintering temperature for 0.3%Pt/Al2O3 and 0.6%Pt/Al2O3 catalysts, respectively. The profile after oxidation was higher than that after reduction. The normalized dispersion went through a maximum at 6508C for the 0.3%Pt/Al2O3 catalyst for both the oxidized and reduced catalyst with the oxidized profile peaking at 2.08 compared with 1.58 for the reduced profile. The percentage difference at the peak is 23% for 0.3%Pt/Al2O3 and 0.7% for the 0.6%Pt/Al2O3. The difference narrows as the sintering temperature (or particle diameter) increases for the 0.3%Pt2O3 catalyst. For the 0.6%Pt/Al2O3, the dispersion diverges as from a sintering temperature of 6508C.

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SINTERING-REDISPERSION ON YIELD OF PLATINUM CATALYSTS

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Table 2. Variation of hydrogen uptake, dispersion and particle size with sintering temperature for 0.3%Pt/Al2O3 catalyst. Values within brackets refer to reduced catalysts. Sintering temperature (8C) 500 550 600 650 700 750 800

Hydrogen uptake after oxygen treatment (cm3)

Hydrogen uptake after reduction (cm3)

0.519 0.568 0.487 0.316 0.180 0.162 0.126

0.125 0.145 0.118 0.091 0.057 0.046 0.034

Dispersion (D) 0.45 0.50 0.42 0.28 0.16 0.14 0.11

(0.33) (0.39) (0.31) (0.24) (0.15) (0.12) (0.09)

Normalized dispersion (D/Do) 1.88 2.08 1.75 1.17 0.67 0.58 0.46

(1.38) (1.58) (1.29) (1.00) (0.63) (0.50) (0.38)

Particle diameter (nm) 2.21 2.02 2.36 3.64 6.37 7.09 9.56

(3.03) (2.60) (3.23) (4.17) (6.67) (8.34) (11.11)

Dispersion on fresh 0.3%Pt/Al2O3 catalyst, Do ¼ 0.24.

A percentage difference of 39.25% has also been reported for the Pt/Al2O3 catalyst where alternate oxidation and reduction process were investigated during multiple deactivation and regeneration schemes (Wanke and Bolivar, 1981). Dispersion increased with hydrogen uptake. The normalized dispersion at a sintering temperature of 5508C was found to go through a maximum of 1.058 for the 0.3%Pt/Al2O3 catalyst while it is 1.608 for the 0.6%Pt/ Al2O3 catalyst after sintering in oxygen. This is an indication of a higher level of redispersion on the 0.6%Pt/ Al2O3 catalyst. There was a drop in the hydrogen uptake after reduction when compared with the result obtained after oxidation for the Pt/Al2O3 catalyst. This accounts for the higher dispersion after oxidation over that after reduction for the same catalyst. Results also show that the particle diameter increased with oxygen sintering temperature. The particle diameter after oxidation ranged between 2.21 nm and 9.80 nm for the 0.3%Pt/Al2O3 catalyst while it lay between 2.76 and 4.87 for the 0.6%Pt/ Al2O3 for sintering temperatures between 500 and 8008C. However, after reduction, the particle size ranged between 2.60 nm and 11.11 nm for the 0.3%Pt/Al2O3 catalyst and between 3.36 nm and 8.93 nm for the 0.6%Pt/Al2O3 catalyst within the same temperature range.

Pt-Re/Al2O3 catalyst Table 4 shows the summary of the results for the chemisorption experiments on the Pt-Re/Al2O3 catalyst and Figure 4 indicates that the normalized dispersion profile for the Pt-Re/Al2O3 catalyst which were clearly above unity for both reduced and oxidized catalyst surfaces at

all temperatures investigated. This is contrary to the profiles shown in Figures 2 and 3 where the profiles were above unity at temperatures 6508C for 0.3%Pt/Al2O3 and 0.6%Pt2O3 catalysts. This is indicative of an increased dispersion (redispersion) at all sintering temperatures after the oxidation and reduction of the Pt-Re/Al2O3. Also, the dispersion values after reduction are lower than those obtained after oxidation. At the maximum (5508C), the value after oxidation is higher than that after reduction by 14.3%. The dispersions on the bimetallic catalyst are higher than those on the monometallic catalysts. More importantly, the dispersion at the highest sintering temperature (8008C) was 0.298, more than twice the value on the 0.3%Pt/Al2O3 catalyst.

Observations The results of the chemisorption experiments reveal that the Pt-Re/Al2O3 bimetallic catalyst is redispersed at all sintering conditions investigated (Figure 4). The normalized dispersion gives a reliable measure of the extent of sintering as long as the adsorption stoichiometry is independent of crystallite size. Catalysts with normalized dispersion above unity are redispersed since the dispersion after treatment exceeds the dispersion on the fresh surface. When the normalized dispersion is below unity, the catalyst is said to have sintered or deactivated. In this case, there has been loss of the catalyst surface area due to agglomeration of the surface crystallites leaving fewer crystallites on the catalyst surface. Figure 2 also reveals that the 0.3%Pt/Al2O3 catalyst redisperses in the temperature range of 500 –6508C and sinters at temperatures higher than 6508C. This is in

Table 3. Variation of hydrogen uptake, dispersion and particle size with sintering temperature for 0.6%Pt/Al2O3 catalyst. Values within brackets refer to reduced catalysts. Sintering temperature (8C) 500 550 600 650 700 750 800

Hydrogen uptake after oxygen treatment (cm3)

Hydrogen uptake after reduction (cm3)

0.663 0.680 0.640 0.489 0.471 0.389 0.388

0.194 0.228 0.194 0.138 0.112 0.110 0.086

Dispersion (D) 0.29 0.30 0.28 0.22 0.21 0.17 0.17

(0.25) (0.30) (0.25) (0.18) (0.15) (0.14) (0.11)

Normalized dispersion (D/Do) 1.58 1.61 1.52 1.16 1.12 0.93 0.92

(1.36) (1.60) (1.38) (0.97) (0.78) (0.77) (0.60)

Dispersion on fresh 0.6%Pt/Al2O3 catalyst, Do ¼ 0.19.

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Particle diameter (nm) 2.85 (3.94) 2.76 (3.36) 2.76 (3.95) 3.87 (5.56) 4.02 (5.85) 4.86 (6.94) 4.872 (8.93)

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Figure 2. Normalized dispersion profiles as a function of sintering temperature for 0.3%Pt/Al2O3 catalyst.

agreement with the results on 0.5%Pt/Al2O3 catalyst from an earlier study by Wanke (1981) (see Figure 3). These authors reported that redispersion occurs at temperatures between 5008C and 6008C. The dispersion after oxidation was higher than the dispersion after reduction at the temperatures investigated in conformity with our results. The dispersion measurements for the 0.3%Pt/Al2O3 catalyst were found to be higher than those on the 0.6%Pt/ Al2O3 catalyst at different temperatures of oxygen treatment. Both catalysts have the same characteristic but the only difference is the metal loading. Graham and Wanke (1981) reported that three catalysts with the same support and pretreatment but different metal loading (0.53%, 1.0% and 4.0% Pt supported on Kaiser KA-201 alumina spheres) gave different dispersion values at different oxygen sintering temperatures in the order 0.53%Pt . 1.0%Pt . 4.0%Pt. Flynn and Wanke (1975) had proposed the interparticle model to explain the

Figure 3. Normalized dispersion profiles as a function of sintering temperature for 0.6%Pt/Al2O3 and 0.5% Pt/Al2O3 catalysts (Wanke, 1981).

observed trend with metal loading. The interparticle transport model predicts a decreasing dispersion as the metal loading increases. For high metal loading, the rate of sintering is controlled by loss of particles only and rise in dispersion due to the number of migrating surface species that approaches zero is negligible. The number of surface species is said to approach zero because in the model high metal loading corresponds to high velocity of migration. For lower metal loading with reduced velocity of migrating species, the amount of free surface atoms builds up leading to substantially higher dispersions and reduced overall rates of sintering. Wanke (1981) had reported the dispersion of a bimetallic catalyst was not the average of the two monometallic catalysts. This is a clear indication that the metals in the bimetallic system are present as bimetallic clusters. An extended treatment in oxygen results in segregation of the bimetallic clusters into monometallic clusters (Sinfelt and Via, 1979; Kluksdahl, 1968). The sintering behaviour obtained after the segregation is similar to that obtained for a physical mixture of the two monometallic catalysts. Also, in hydrogen atmosphere, this segregation does not occur and the bimetallic catalyst behaves like the monometallic catalyst. However, our work on the bimetallic Pt-Re/Al2O3 catalyst shows that the catalyst redisperses in oxygen atmosphere and sinters in hydrogen atmosphere even though the dispersions after both oxygen and hydrogen treatments are higher than the measured dispersion on the fresh catalyst surface. This redispersion in both oxygen and hydrogen atmospheres in relation to the stabilizing effect of segregated Re on the Pt oxide support complex made sintering of the bimetallic catalyst difficult even at high temperatures. This view is supported by Kluksdahl (1968) who reported that the particle size of Pt is reduced by Re inhibiting sintering. The lowering of dispersion observed during hydrogen treatment is supported by the work of Den Otter and Dautzenberg (1978). However, the authors argued that this lowering of dispersion due to decrease in hydrogen uptake might not be the true representation of the state of the catalyst. They found that during heat treatment of Pt/ Al2O3 catalysts in hydrogen at temperatures above 4978C certain portions of the surface platinum atoms become ‘inaccessible’ and causes a decrease in H/Pt ratio. For instance, 60% Pt was rendered ‘inaccessible’ to hydrogen at 6348C for 10 h. An examination by electron microscopy shows no indication of any change in particle size during pretreatment with hydrogen. They suggested that the ‘inaccessible’ part of the platinum might have resulted from a reversible surface combination between reduced platinum and reduced aluminum to form some sort of superficial alloy. A mild oxidation at 394– 4948C could restore the catalyst back to its original state with all the surface platinum atoms once again becoming accessible for chemisorption by hydrogen. Graham and Wanke (1981) however disagreed with the above result. They measured H2 uptakes on fresh samples of Pt and Ir supported on alumina after which the catalysts were treated in hydrogen or oxygen at elevated temperatures. The temperature range for treatments in oxygen was 300– 6008C and sintering in hydrogen was done at 6508C and 8008C. They did not observe any decrease in H/Pt ratios of the magnitude reported by Otter and Dautzenberg (1978) for treatment of Pt/Al2O3 catalyst in

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Table 4. Variation of hydrogen uptake, dispersion and particle size with sintering temperature for 0.3%Pt–0.3%Re/Al2O3 catalyst. Values within brackets refer to reduced catalysts. Hydrogen uptake after oxygen treatment (cm3)

Hydrogen uptake after reduction (cm3)

Sintering temperature (8C)

258C

5008C

258C

5008C

500 550 600 650 700 750 800

0.189 0.300 0.214 0.214 0.180 0.165 0.133

0.240 0.315 0.295 0.259 0.195 0.190 0.174

0.018 0.0073 0.006 0.043 0.016 0.047 0.034

0.073 0.093 0.091 0.068 0.062 0.056 0.046

Average dispersion (D) 0.41 0.55 0.51 0.45 0.34 0.33 0.30

(0.37) (0.48) (0.47) (0.36) (0.32) (0.30) (0.24)

Average normalized dispersion (D/Do) 1.88 2.53 2.34 2.05 1.56 1.57 1.36

(1.69) (2.17) (2.12) (1.63) (1.44) (1.37) (1.10)

Particle diameter (nm) 2.43 1.81 1.95 2.23 2.92 3.03 3.36

(2.69) (2.11) (2.15) (2.80) (3.17) (3.34) (4.17)

Average dispersion on fresh 0.3%Pt–0.3%Re catalyst, Do ¼ 0.22.

hydrogen at temperatures less than 6008C. They therefore concluded that the discrepancy in the results may be due to the temperatures of 2788C and 08C at which they measured hydrogen uptake and to the relatively low reduction temperature of 4008C which could be sufficient for the removal of adsorbed hydrogen from catalyst that have not been treated in hydrogen at elevated temperatures. Support for this argument came from Menon and Froment (1978) who reported that the desorption of hydrogen becomes more difficult after Pt/Al2O3 catalysts have been exposed to high temperatures; they also found that the changes in hydrogen desorption behaviour are not due to metal-support interaction. While it is difficult to confirm any of the proposals reviewed for explaining redispersion, we can state the possibility of the existence of a temperature threshold below which a metal-support interaction leading to a platinum oxide-alumina complex is unlikely. Hence, at temperatures below this limit, redispersion prevails. Therefore, it is conceivable that the most crucial role of rhenium in a bimetallic platinum-rhenium system is stabilizing this complex at much higher temperatures beyond the limit denoted above. The mechanism for the sintering-redispersion process is still open for debate and further work needs to be done to

Figure 4. Normalized dispersion profiles as a function of sintering temperature for 0.3%Pt–0.3%Re/Al2O3 catalyst.

confirm unambiguously a theory that explains the sinteringredispersion phenomenon. Aromatization Computer simulation Tables 2 –4 show dispersion patterns and corresponding particle size ranges at different sintering temperatures for 0.3%Pt/Al2O3, 0.6%Pt/Al2O3 and 0.3%Pt-0.3%Re/Al2O3 catalysts, respectively. In Table 2 (0.3%Pt/Al2O3), it was clear that the dispersion increased with sintering temperature in O2 and after reduction in H2 from 5008C to 5508C, and decreased thereafter from 5508C to 8008C. The higher dispersion at 5508C has implications for aromatisation selectivity. Also shown in Table 2 is the redispersion pattern where the catalyst was observed to redisperse on the sintered catalyst surface only at temperatures between 500– 6508C; the threshold was 6008C for the reduced catalyst surface. In Table 3 (0.6%Pt/Al2O3), there was a slight increase at 5508C in comparison with the value recorded for the 0.3%Pt/Al2O3 catalyst (0.01 versus 0.05). And thereafter, the dispersion decreased with increasing sintering temperature. However, on the O2-sintered catalyst surface, redispersion persisted on this catalyst up to 7008C. The threshold of redispersion for the reduced surface was 6008C. In Table 4, it is noted that the dispersion increased more substantially from 5008C to 5508C on both O2 sintered and reduced catalyst surfaces. On the bimetallic catalyst surface, the catalyst redisperses at all temperatures investigated for both O2 sintered and reduced surfaces. The discussions that follow will focus on the particle size distribution (PSD) on the working catalyst surface. This working surface is the O2 sintered catalysts at temperatures between 500 – 8008C with subsequent reduction in H2 at 5008C. Particle size distribution (PSD) The discussions on the PSD are useful for three main reasons. First, the smaller the particle diameters, the more selective the catalyst becomes for aromatization. Unfortunately, also, the more hydrogenolytic it becomes; this hydrogenolytic activity might be weak for demethylation reactions, or, it might be very strong, for complete

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degradation reactions. That very small crystallites also promote hydrogenolysis is supported by the work of Sarkany (1989). We will try to distinguish between the three types of sites in this category: aromatization sites, weak and strong hydrogenolytic sites. Secondly, even when a favourable PSD for aromatization is presented, the issue of the narrowness of spread of the PSD might become significant. This is because a narrow spread might preclude the right configurational adsorption of n-heptane and subsequent aromatisation. The configurational question might thus favour hydrogenolysis as opposed to aromatization. Thirdly, in a case whereby conditions of size and spread are favourable, there is still no guarantee that aromatization selectivity would be favoured. This may be due to unfavourable surface configuration of the metal crystallites. The metal crystallites might contain aggregates of Pt atoms that are not compliant with other factors that have been established for aromatization. These other factors are the following: (1) electronic structure of platinum aggregates which has been shown to account for cyclic type mechanism in hydrocarbon intermediate (Legare and Garin, 2003); (2) for bimetallic catalysts, chemisorbed sulfur atoms have been shown to divide the metal surface of platinum-containing catalysts into ensembles of small number of contiguous platinum atoms (Biloen et al., 1980); this is not a factor here as there was no sulfur input in the feed and the sulfur content on the bimetallic catalyst is unsubstantial (50 ppm); (3) geometry of adsorption indicating that cyclization and polymerization are equally facilitated by catalyst configuration of three-atom ensembles of triangular geometry (Paal, 1980, 2003); and (4) the presence of the (111) facets on the cuboctahedra structure of the Pt/SiO2 catalysts (Gnutzmann and Vogel, 1990) provides suitable sites for aromatization. Bond and Paal (1992) had attributed aromatisation to the ‘6-atom (111) planes suitable to form benzene ring situated parallel to the crystal plane’ in a review of recently published works on the 6% Pt/SiO2 catalyst (EUROPT-1). We shall look at the PSD on these catalysts with the objective of deciding how the above categorizations affect aromatisation selectivity. Before further discussion of the catalytic activity behaviour, we need to ensure that the experimental data are free of diffusional intrusions. Our kinetic data gathering was first determined to be free of mass and heat transfer resistances by traditional methods (Susu and Ako, 1984, 1985), and secondly, by the more rigorous criterion of Koros—Nowark (K-N) (Koros and Nowark, 1967). In order to certify that the experimental data satisfied the K-N criterion, two catalyst loadings were used in data gathering: 0.3% and 0.6% Pt/Al2O3. It was shown that with the two-fold change of catalyst loading at comparable particle diameter (4 nm), the turnover numbers (TONs) were approximately equal to 0.09 s21. So, the experimental data were confirmed to be in the kinetic regime as the more rigorous K-N criterion was satisfied. 0.3% Pt/Al2O3 catalyst Figure 5 shows the simulated PSD on the 0.3%Pt/Al2O3 catalyst. Consider first the PSD for the O2 sintered catalyst. The PSD on the fresh catalyst was narrow and congregated between 2.9 and 3.8 nm. By the above discussion, these values of the PSD could equally promote aromatisation

Figure 5. Simulated PSD on 0.3%Pt/Al2O3 catalyst. (a) Simulated PSD on O2 sintered (500, 550, 6008C) and reduced (5008C) 0.3%Pt/Al2O3 catalyst; (b) simulated PSD on O2 sintered (650, 700, 750, 8008C) and reduced (5008C) 0.3%Pt/Al2O3 catalyst.

and hydrogenolysis. The product patterns indicate that it does, as benzene yield from the demethylation of toluene was very prevalent. This would suggest that only weak hydrogenolysis sites (demethylation sites) were available on the catalyst surface. The strong hydrogenolysis sites that would have resulted in the deep fragmentation of nheptane to methane were negligible. In support of the above argument, the product yields on the ‘clean’ catalyst surface were 0.89 for benzene, 0.05 for toluene and 0.06 for methane. It is quite clear that if the values of the PSD and spread are determinants of aromatisation, then, the values recorded at 5008C are adequate for good aromatization selectivity. It is also clear that high demethylation sites are prevalent here as toluene, the primary aromatization product, was almost completely demethylated to benzene. As the methane content in the gas phase is so small (0.06), we cannot invoke the results of Sarkany (1988, 1989) in explaining the location of excess methane production. Sarkany (1988, 1989) had explained the production of excess methane in the gas phase as being due to the H2 split of Pt-C ensembles by hydrogen. In any case, the catalysis here occurs on a ‘clean’ catalyst surface. Where then is the methane produced from toluene demethylation? As previously reported elsewhere, the excess methane might be occluded between the platinum

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SINTERING-REDISPERSION ON YIELD OF PLATINUM CATALYSTS metals on the catalyst surface, accounting for even higher dispersion of platinum atoms on the catalyst surface. Such a process was shown to promote aromatization, especially in Pt-Re/Al2O3 catalyst (Aberuagba and Susu, 1999). At a sintering temperature of 5008C, the PSD shifted to the left and the range of particle diameters was 1.2– 3.0 nm. The shift was significant in terms of aromatic selectivity as smaller particle diameters now predominate. The spread was however wider, from 0.9 nm on the fresh catalyst to 1.8 nm on the catalyst sintered at 5008C. What are the implications of these results on aromatic selectivity? On the ‘clean’ sintered catalyst surface, toluene demethylation to benzene predominated on the ‘clean’ surface (0.89). On the ‘fresh’ catalyst surface, however, only toluene was formed; demethylation sites were completely absent. It is then clear from these results that the shift to smaller particle diameters results in the production of selective toluene sites. We could, of course, specify the threshold of the particle diameter for selective toluene production not better than ,3 nm. The results here clearly delineate the characteristics of hydrogenolic sites from the aromatic sites for small particle sizes. The narrower the spread, the more hydrogenolytic the sites become; also, the narrower the spreads, the stronger the hydrogenolytic sites. The PSD at 5508C became narrower still than that at 5008C. The spread in this case was from 1.0 –2.5 nm, a spread of 1.5 nm. Here, the catalyst was strongly hydrogenolytic (0.73); benzene production (0.23) indicates that demethylation sites were also available, confirming the above classification of sites. It is therefore possible that the cracking sites that are responsible for demethylation are much weaker sites than those responsible for total degradation of n-heptane to methane (nC7 ! 7C1). At 6008C, the PSD spread and range is quite close to that at 5008C. How then does the product distribution profiles compare? In this case, we have complete degradation of n-heptane to methane, totally contrary to the case on sintered (5008C) catalyst, where only toluene was produced. The explanation here is the lack of the three-atom ensembles of triangular geometry that have been shown to be necessary for aromatic selectivity (Paal, 2003) at the higher sintering temperature. The occurrence of the threeatom ensembles can indicate the presence of facets with (111) symmetry produced during sintering and redispersion (Bond and Paal, 1992; Gnutzmann and Vogel; 1990). Nevertheless, prolonged pulsing of the catalyst surface at 6008C yielded more and more benzene, such that at the 50th pulse, the mole fraction of methane and benzene were 0.72 and 0.28, respectively. This would suggest the impartation of aromatisation sites assisted by coking, through the availability of ‘Pt-C-H’ entities (Sarkany, 1988, 1989). At a sintering temperature of 6508C, the surface of the catalyst only supported the predominant production of methane (0.97), although, again, coking imparted some aromatization activity on the catalyst; again, at the 50th pulse, the yields of methane, benzene and toluene were 0.71, 0.13 and 0.16, respectively. At 7008C, where the PSD ranged between 2.2 and 7.0 nm, the sintered catalyst surface yielded a mixture of hydrogenolytic and aromatisation sites (0.64 methane and 0.23 toluene), and no demethylation sites for the production

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of benzene. At 7508C, the catalyst was completely hydrogenolytic as only methane was produced. There was no reaction on the catalyst sintered at 8008C. A summary of the simulation studies on 0.3% Pt/Al2O3 catalyst could be stated thus. At lower sintering temperatures (,6508C), where there were available particle diameters in the low range responsible for demethylation and hydrogenolytic sites. Sintering temperature at 5008C lead to the predominant production of toluene, an indication that the right three-atom structure of Pt atoms with the formation of (111) type facets during sintering and redispersion, are predominant here. At sintering temperatures between 650 – 7508C, hydrogenolysis was predominant and no reaction occurred at 8008C. 0.6% PtAl2O3 catalyst The reforming experiments carried out on sintered 0.6% Pt/Al2O3 catalyst at 5008C and 6008C, yielded only methane from deep fragmentation of n-heptane. No aromatisation products were produced. However, on the fresh catalyst surface, the yields of methane, benzene and toluene were 0.72, 0.18 and 0.10, respectively. In comparison, at 5008C and 6008C on sintered 0.3% Pt/Al2O3 catalyst, aromatization activity was very much evident, and this is in spite of the fact that the simulation PSD on the 0.6% Pt/ Al2O3 catalyst was quite similar to those on the 0.3% Pt/ Al2O3 catalyst. The absence of aromatics on the sintered 0.6%Pt/Al2O3 catalyst in spite of the fact that the particle size range was well in the region for aromatic production, lends credence to the suggestion that a preferred surface structure is required for aromatization. We had pointed out earlier that the necessity for a favourable configuration for aromatization to occur, besides the need for a favourable particle size range. In support of this argument is the need for a triangular arrangement of aggregates of active Pt atoms for aromatisation (Paal, 1980) with the presence of (111) facets in a cuboctahedra structure during sintering and redispersion (Poltorak and Boronin, 1966; Gnutzmann and Vogel, 1990; Bond and Paal, 1992). What this implies is that this triangular arrangement and the presence of (111) facets, are least favoured on the sintered 0.6%Pt/Al2O3 catalyst, and most favourable on the 0.3%Pt/Al2O3 catalyst. How do all these results on the monometallic catalysts, especially the 0.3%Pt/Al2O3 catalyst, compare with the results on the bimetallic 0.3%Pt – 0.3%Re/Al2O3 catalyst? The difference on the fresh catalyst is quite clear and well documented. Only methane was produced on the bimetallic catalyst while benzene and toluene were produced to varying degrees on the monometallic catalyst. The results on the sintered bimetallic catalyst are very striking and may provide extra explanations for the superiority of the bimetallic catalyst. Pt-Re/Al2O3 catalyst Figure 6 shows the simulated PSD on the 0.3%Pt – 0.3%Re/Al2O3 catalyst. At the lowest sintering temperature of 5008C, the PSD lay between 1.5– 2.5 nm, a spread of only 1.0 nm, which only supports methane production. This has been attributed to the segregation of Re on the surface (Susu and Ogogo, 2005). Re is a strong hydrogenolytic metal, although by our new categorization, it could also be that the PSD was too narrow for adsorption and

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SUSU et al. 2.5 nm. The bimetallic catalyst became wholly aromatization selective, with complete production of toluene. In summary, the bimetallic catalyst maintained a much narrower spread of particle diameters than the monometallic catalyst at all the sintering temperatures investigated. Even on a catalyst sintered at 8008C, where no reaction was observed on the 0.3%Pt/Al2O3 catalyst, the bimetallic catalyst was a complete aromatization catalyst, without any weak or strong hydrogenolytic sites for either demethylation to benzene or total fragmentation to methane. It is this property of the maintenance of desirable particle diameters in the face of harsh sintering conditions that further explains the superiority of the Pt-Re/Al2O3 catalyst over its monometallic counterpart.

AROMATIZATION SELECTIVITY There is need to support the above discussions with the presentation of data on the selectivity for aromatics on sintered 0.3%Pt/Al2O3 and 0.3%Pt – 0.3%Re/Al2O3 catalysts during n-heptane reforming. Aromatic selectivity is defined as follows (Bond, 1987; Paal, 1995): SB ¼ P

benzene (aromatics) þ methane

ST ¼ P

toluene (aromatics) þ methane

and

Figure 6. Simulated PSD on 0.3%Pt–3%Re/Al2O3 catalyst. (a) Simulated PSD on O2 sintered (500, 550, 6008C) and reduced (5008C) Pt-Re/Al2O3 catalyst; (b) simulated PSD on O2 sintered (650, 700, 750, 8008C) and reduced (5008C) Pt-Re/Al2O3 catalyst.

aromatisation. We must also consider the inability of the sintered bimetallic surface to provide the three-atom, triangular arrangement of Pt aggregates, in the presence of Re. At 5508C, not only was there a significant shift to the left (1.2 –1.8 nm), the spread was even narrower (0.6 nm). By our earlier deduction that very small particle sizes and narrower spreads should result in a more hydrogenolytic catalyst, we find here that, contrary to this expectation, some aromatisation was evident: methane (0.73), benzene (0.21) and toluene (0.06). At 6008C, the spread of the PSD was 1.0 nm as opposed to 0.6 nm at 5508C and 1.0 nm at 5008C. Yields of products at this temperature were 0.37, 0.42 and 0.21 for methane, benzene and toluene, respectively, indicating less methane and more aromatization. The comparable yields at 5508C were 0.73, 0.21 and 0.06 for methane, benzene and toluene. And at 5008C, only methane was formed. There appears to be no correlation whatsoever between the spread, the particle diameters and aromatization selectivity. But spectacularly at 8008C, where the particle diameters for the 0.3%Pt/Al2O3 and 0.6%Pt/Al2O3 catalyst were as high as 11.7 and 9.5 nm, respectively, the particle diameter remained small (1.7 to 4.2 nm), representing a spread of

Tables 5 and 6 display the aromatic selectivity values at the first and 25th pulse of n-heptane at different sintering temperatures for the 0.3%Pt/Al2O3 and 0.3%Pt – 0.3%Re/ Al2O3 catalysts, respectively. In both tables, the displays are on catalysts that were sintered at the indicated temperatures. The first pulse gives an indication of the product distribution on a ‘clean’ surface, while on the 25th pulse, the reaction occurs on a coked surface. The first significant observation here is the superiority of the aromatic selectivity values for the Pt-Re/Al2O3 catalyst, which ordinarily is a hydrogenolytic catalyst unless in the presence of sulphur or other surface modifiers. In this work, the enhanced aromatic selectivity was observed only after sintering and redispersing at temperatures between 500 –8008C.

Table 5. Aromatic selectivity on the first and 25th pulse of n-heptane on sintered 0.3%Pt/Al2O3 catalyst at different sintering temperatures. 1st pulse Sintering temperature (8C) 500 550 600 650 700 750 800

25th pulse

Benzene selectivity (SB)

Toluene selectivity (ST)

Benzene Selectivity (SB)

Toluene selectivity (ST)

0.89 0 0.36 0 0 0 0

0.11 0 0. 0.03 0.26 0 0

1 0 0 0.17 0.74 0 0

0 0.36 0.80 0.76 0.26 0 0

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SINTERING-REDISPERSION ON YIELD OF PLATINUM CATALYSTS Table 6. Aromatic selectivity on the first and 25th pulse of n-heptane on sintered 0.3%Pt–0.3%Re/Al2O3 catalyst at different sintering temperatures. 1st pulse Sintering temperature (8C) 500 550 600 650 700 750 800

25th pulse

Benzene selectivity (SB)

Toluene selectivity (ST)

Benzene selectivity (SB)

Toluene selectivity (ST)

0 0.21 0.42 0.12 0.65 0.56 0

0 0.06 0.21 0.04 0 0 1

0.10 0.57 0.34 0.73 0.84 0.57 0

0.31 0.11 0.22 0 0 0.11 1

As indicated in Table 5, at a sintering temperature of 5008C, on ‘clean’ 0.3%Pt/Al2O3 catalyst surface, the selectivity for benzene was 0.89 (89%) while the corresponding value for toluene was 0.11 (11%). There were no strong hydrogenolytic sites for total fragmentation of n-heptane to methane on this surface. However, the situation on the coked surface of the monometallic catalyst at the same temperature (5008C) was dramatically different. In this case, only dealkylation sites predominated as only benzene was produced to the total exclusion of toluene and methane. The fact that aromatization was enhanced on the coked surface has been stated and discussed above. Table 6 shows an interesting departure from the behaviour on the monometallic catalyst surface. First, in the case of the bimetallic catalyst at a sintering temperature of 8008C, toluene was the only product on both the ‘clean’ and coked surfaces, implying a catalyst of cuboctahedral structure with predominant facets of (111) symmetry. In view of the results on the Pt/Al2O3 catalyst, the facets of (100) symmetry must be responsible for toluene demethylation to benzene, while facets of (111) symmetry must be responsible for toluene production. So, the fact that benzene was absent on the Pt-Re/Al2O3 catalyst would imply the absence of facets of (100) symmetry. Could we account for the deep fragmentation sites on these catalysts? We are assuming here that the corner atoms are responsible for deep fragmentation to methane. Since sintering is at high temperatures, we further suggest that the sintering process would round the particles leading to the reduction in the number of corner atoms; this would then account for the removal of deep fragmentation sites. We note that the selective production of toluene occurs after the bimetallic catalyst has been sintered at 8008C, a temperature where the monometallic catalyst yielded no reaction. As pointed out earlier, this again, confirmed the superiority of the bimetallic (Pt –Re) catalyst over the monometallic catalyst. CONCLUSIONS In this work, the catalysis of n-heptane on sintered platinum-containing catalysts was used for further understanding of the superiority of the bimetallic catalyst over its monometallic counterpart. First, the size relatedness of the sintering process was investigated by use of the

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atomic migration model for the prediction of the PSD for the monometallic and bimetallic catalysts. Additionally, the importance of the presence of three-atom ensembles of triangular symmetry with subsequent formation of facets with (111) symmetry on sintering and redispersion, was invoked for aromatic selectivity. Significantly, the formation of benzene and toluene on the 0.3% Pt/Al2O3 catalyst occurs at 5008C, devoid of deep fragmentation sites for methane production. On the ‘clean’ and coked 0.3%Pt – 0.3%Re/Al2O3 catalyst, toluene was the sole product of reaction at the much higher sintering temperature of 8008C; a temperature at which no reaction occurred on the 0.3%Pt/Al2O3 catalyst. We believe that this attribute of the Pt –Re catalyst provides additional evidence for the superiority of the bimetallic catalyst over its monometallic counterpart. We suggest that this special attribute can be linked to the predominance of facets with (111) symmetry on the bimetallic catalyst surface after sintering at a temperature as high as 8008C. As far as the authors are aware, this is the first time that sintering has been used to support this very important attribute of bimetallic catalyst behaviour. Another significant result is the fact that aromatization was absent on sintered 0.6% Pt/Al2O3 catalyst, although the PSD is in the right range for aromatization. This is clear evidence that the preferred cuboctahedra structure of 6-atom (111) planes formed after sintering does not occur on the 0.6% Pt/Al2O3 catalyst surface.

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ACKNOWLEDGEMENT The authors acknowledge with gratitude the incisive comments of the reviewers, and for especially bringing to our attention more relevant literature on the surface structure of the supported platinum catalyst. The manuscript was received 13 August 2004 and accepted for publication after revision 2 May 2006.

Trans IChemE, Part A, Chemical Engineering Research and Design, 2006, 84(A8): 664– 676